1. Field of the Invention
The present invention generally relates to a position location system and, more particularly, to using long term satellite tracking data in a remote receiver.
2. Description of the Related Art
Global Positioning System (GPS) receivers use measurements from several satellites to compute position. GPS receivers normally determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide the distance from the receiver to each of the satellites that are in view of the receiver. The GPS satellites transmit to the receivers satellite-positioning data, so called “ephemeris” data. In addition to the ephemeris data, the satellites transmit to the receiver absolute time information associated with the satellite signal, i.e., the absolute time signal is sent as a second of the week signal. This absolute time signal allows the receiver to unambiguously determine a time tag for when each received signal was transmitted by each satellite. By knowing the exact time of transmission of each of the signals, the receiver uses the ephemeris data to calculate where each satellite was when it transmitted a signal. Finally, the receiver combines the knowledge of satellite positions with the computed distances to the satellites to compute the receiver position.
More specifically, GPS receivers receive GPS signals transmitted from orbiting GPS satellites containing unique pseudo-random noise (PN) codes. The GPS receivers determine the time delays between transmission and reception of the signals by comparing time shifts between the received PN code signal sequence and internally generated PN signal sequences.
Each transmitted GPS signal is a direct sequence spread spectrum signal. The signals available for commercial use are provided by the Standard Positioning Service. These signals utilize a direct sequence spreading signal with a 1.023 MHz spread rate on a carrier at 1575.42 MHz (the L1 frequency). Each satellite transmits a unique PN code (known as the C/A code) that identifies the particular satellite, and allows signals transmitted simultaneously from several satellites to be received simultaneously by a receiver with very little interference of any one signal by another. The PN code sequence length is 1023 chips, corresponding to a 1 millisecond time period. One cycle of 1023 chips is called a PN frame. Each received GPS signal is constructed from the 1.023 MHz repetitive PN pattern of 1023 chips. At very low signal levels, the PN pattern may still be observed, to provide unambiguous time delay measurements, by processing, and essentially averaging, many PN frames. These measured time delays are called “sub-millisecond pseudoranges”, since they are known modulo the 1 millisecond PN frame boundaries. By resolving the integer number of milliseconds associated with each delay to each satellite, then one has true, unambiguous, pseudoranges. The process of resolving the unambiguous pseudoranges is known as “integer millisecond ambiguity resolution”.
A set of four pseudoranges together with the knowledge of the absolute times of transmissions of the GPS signals and satellite positions at those absolute times is sufficient to solve for the position of the GPS receiver. The absolute times of transmission are needed in order to determine the positions of the satellites at the times of transmission and hence to determine the position of the GPS receiver. GPS satellites move at approximately 3.9 km/s, and thus the range of the satellite, observed from the earth, changes at a rate of at most ±800 m/s. Absolute timing errors result in range errors of up to 0.8 m for each millisecond of timing error. These range errors produce a similarly sized error in the GPS receiver position. Hence, absolute time accuracy of 10 ms is sufficient for position accuracy of approximately 10 m. Absolute timing errors of much more than 10 ms will result in large position errors, and so typical GPS receivers have required absolute time to approximately 10 milliseconds accuracy or better.
It is always slow (no faster than 18 seconds), frequently difficult, and sometimes impossible (in environments with very low signal strengths), for a GPS receiver to download ephemeris data from a satellite. For these reasons, it has long been known that it is advantageous to send satellite orbit and clock data to a GPS receiver by some other means in lieu of awaiting the transmission from the satellite. This technique of providing satellite orbit and clock data, or “aiding data”, to a GPS receiver has become known as “Assisted-GPS” or A-GPS.
In one type of A-GPS system, the GPS receiver measures and transmits pseudoranges to a server and the server locates position of the GPS receiver. Such a system is referred to herein as a “mobile-assisted” system. In a mobile-assisted system, for each position computation, there are four transactions between the GPS receiver and the server: a request for assistance from the receiver to the server, transmission of aiding information from the server to the receiver, transmission of pseudorange measurements from the receiver to the server, and finally transmission of position from the server to the receiver. In most mobile-assisted systems, a new request and new aiding information are sent for each new position, since the assistance data is only valid for a short period of time (e.g., minutes). Thus, for mobile-assisted systems, the total time to fix position is deleteriously affected by the number of transactions between the receiver and the server. In addition, if the receiver roams beyond the service area of the network that delivers the assistance data, the receiver must acquire satellite signals and compute position autonomously, assuming the receiver is even capable of autonomous operation.
In another type of A-GPS system, the GPS receiver locates its own position using assistance data from a server. Such a system is referred to herein as a “mobile-based” system. In a mobile-based system, for each position computation, there are up to two transactions between the receiver and the server: the receiver requests assistance from the server and the server sends aiding information to the receiver. The position is computed inside the receiver using the aiding information. In conventional mobile-based systems, the aiding information is ephemeris data valid between 2 to 4 hours. That is, the ephemeris data is the same data as broadcast by the satellites. Thus, for conventional mobile-based systems, the total time to fix position may be deleteriously affected if the receiver must compute position outside of the 2-to-4 hour period during which the aiding data is valid, since further transactions between the receiver and the server are required. In addition, if the receiver roams beyond the service area of the network that delivers the assistance data for a period longer than 2-to-4 hours, the receiver must acquire satellite signals and compute position autonomously.
Therefore, there exists a need in the art for a method and apparatus that uses satellite tracking data in a remote receiver in a manner that minimizes the number of transactions between the receiver and a server and allows for extended operation outside of the service area of the network.